DE 10 2009 000 857 A1 discloses a plug-in pump for a fuel injection system. The plug-in pump has a piston, which is arranged in a cavity of a cylinder head and is driven by a camshaft via a roller tappet. The piston can be moved in a linear fashion in the cavity in order to open an inlet for the fuel into a pump working chamber in a suction stroke. During a subsequent delivery stroke, the fuel is passed out of the pump working chamber through a pump outlet to another unit of the engine. At its end remote from the pump working chamber, the pump piston has a sealing element surrounding it, which seals off the piston with respect to the engine oil, said oil lubricating the camshaft, for example, in order to prevent fuel from being able to get into the engine oil and vice versa.
Conventional seals, e.g. combination seals of the kind known in the prior art, are often incapable of preventing fuel from getting into the engine oil and vice versa with sufficient reliability, and therefore it is not always possible to meet the demands made by engine designers on such pumps.